perf(marketplace): stream SHA256 + zip extract (no dual full buffers)

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

SafeZipExtractor.readBoundedBytes uses file.readAsBytes() (up to 100 MiB compressed). Marketplace/sideload then SHA256 the whole buffer and ZipDecoder().decodeBytes, keeping entry contents (uncompressed limit up to 500 MiB). Peak ≈ compressed + decoded + per-file copies.

Updater already streams SHA256 — marketplace should match.

Evidence

• lib/core/security/safe_zip_extractor.dart (~43–88)
• lib/core/market/http_marketplace_repository.dart (~174–221)
• lib/core/extensions/local_extension_installer.dart

Acceptance

• SHA256 via stream (sha256.bind(file.openRead())) like updater
• Extract without holding compressed + all uncompressed at once (isolate and/or entry streaming)
• Existing zip-bomb limits (security(archive): zip bomb limits and bounded decompression for extension/updater installs #398) still enforced
• Tests for hash stream + oversized archive rejection

Suggested fix

Align with updater checksum streaming; decode/write on a background isolate; write entries incrementally.

[ZhuchkaTriplesix]

Shipped on dev via #433 (Closes did not auto-close because PRs targeted dev).